BJ Penn would have probably done well if he did straight boxing when he started fighting. Rampage probably could have done have decent too although now he's just a straight haymaker thrower now but he could have been good.

Ok well any of the guys who showed great natural talents even though they had only been training boxing for not so long. Still, it would take years and years for the best ones to compete against the top boxers, and I would still doubt any coming close to top 20 in the world. Boxing is way too difficult to just break through.

MMA fighters spend too much time grappling, kicking etc. The only combat sport fighters that went into boxing succesfully were muay Thai and kickboxers, but they already trained a lot of boxing and were very good at it already.

Well naturally there are a few boxing champs who have tried to walk through MMA fighters in their own sport. Their hands are always leaps and bounds ahead of even the most dangerous MMA champions, but their records show that they didn't take the challenge seriously, à la James Toney. Not sure if they can be counted because someone like that isn't committed to the sport.

Though there are a few who have made a decent transition and have good records, I'd say Jeremy Williams, undefeated in MMA at 5-0 should be considered to have some of the best hands of anyone to successfully compete in the sport. He holds a 42(35)-5-1 record in professional boxing. Though he hasn't competed in MMA since 2008.

Outside of folks who have had substantial records in professional boxing, I always thought Marcus Hicks had good form, speed, power, etc. and everything a boxer would need with the exception of a good gas tank.

Just another thought, once there is a decent commission or some type of committee that can regulate the sport of MMA better and give the fighters what they deserve as far as pay goes, we will be seeing more and more dangerous stand up fighters competing. As for now, I still see some Billy Blanks Tae Bo stand up in most of the fights with the exception of a few contenders and champs that posses borderline professional boxing skills.